1. Field of the Invention
The present invention relates to an EGR control system for an internal combustion engine, for controlling high-temperature and low-temperature recirculated gases, particularly to an EGR control system for an internal combustion engine, for controlling internal EGR in which combustion gases generated by combustion are caused to remain in a cylinder, and more particularly to an EGR control system for an internal combustion engine, for controlling the internal EGR by changing lift of exhaust valves by an exhaust valve lift mechanism.
2. Description of the Related Art
Conventionally, an EGR control system for an internal combustion engine, of the above-mentioned kind, has been disclosed e.g. in the publication of Japanese Patent Publication No. 3305416 (first prior art). This engine is of a spark ignition type, and includes a low-temperature EGR device and a high-temperature EGR device. The low-temperature EGR device recirculates relatively low-temperature exhaust gases flowing in an exhaust passage to an intake passage side. The low-temperature EGR device includes a low-temperature EGR passage and a low-temperature EGR control valve. The low-temperature EGR passage connects between an intake passage and the exhaust passage, and in an intermediate portion of the low-temperature EGR passage, there is disposed a low-temperature EGR control valve. The degree of opening of the low-temperature EGR passage is changed, as desired, by the low-temperature EGR control valve, whereby the amount of exhaust gases recirculated through the low-temperature EGR passage (hereinafter referred to as “the low-temperature EGR amount”) is changed.
On the other hand, the high-temperature EGR device recirculates higher-temperature exhaust gases than the exhaust gases recirculated by the low-temperature EGR device, into a cylinder. The high-temperature EGR device includes a high-temperature EGR passage and a high-temperature EGR control valve. The high-temperature EGR passage is formed in a cylinder head of the engine. The high-temperature EGR passage has one end thereof opening into a combustion chamber and the other end thereof opening into an exhaust port, and in an intermediate portion of the high-temperature EGR passage, there is disposed a high-temperature EGR control valve. The degree of opening of the high-temperature EGR passage is changed, as desired, by the high-temperature EGR control valve, whereby the amount of exhaust gases recirculated through the high-temperature EGR passage (hereinafter referred to as “the high-temperature EGR amount”) is changed.
In this EGR control system, the respective ratios of the high-temperature EGR amount and the low-temperature EGR amount to a total EGR amount are determined by searching a map according to the load of the engine. Then, the high-temperature EGR control valve and the low-temperature EGR control valve are duty-controlled based on these ratios, whereby the high-temperature EGR amount and the low-temperature EGR amount are controlled.
Further, a conventional EGR control system for an internal combustion engine, for controlling internal EGR has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2004-251183 (second prior art). This EGR control system is provided with variable valve actuating mechanisms on respective intake and exhaust sides for changing an intake cam phase and an exhaust cam phase, and with these variable valve actuating mechanisms, changes the opening and closing timings of intake valves and exhaust valves to change the valve overlap of the intake and exhaust valves, whereby the internal EGR amount is controlled.
More specifically, an actual overlap area as the area of an actual valve overlap of intake and exhaust valves is calculated, and the variable valve mechanisms are controlled such that the valve overlap area of the intake and exhaust valves becomes equal to a value obtained by multiplying the ratio of a target internal EGR amount to an actual internal EGR amount and the actual valve overlap area by each other.
Further, there has been known another conventional EGR control system for an internal combustion engine, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H05-18324 (third prior art). This EGR control system recirculates part of exhaust gases exhausted from the engine as an EGR gas from the exhaust pipe to the intake pipe via an EGR passage, and an EGR control valve is disposed in an intermediate portion of the EGR passage, for controlling the EGR rate. Further, an oxygen concentration sensor is disposed in the intake pipe at a location downstream of a portion of the intake pipe into which the EGR gas is recirculated. The oxygen concentration sensor detects a concentration of oxygen in intake air mixed with the EGR gas, and a predetermined map is searched according to the detected oxygen concentration, to thereby calculate an actual EGR rate as an EGR rate which is actually applied.
Further, the engine speed and intake manifold pressure are detected, and a target EGR rate is calculated based on the detected values thereof. At the same time, an EGR basic control variable is calculated according to the target EGR rate. Then, a EGR correction value is calculated such that the difference between the calculated target EGR rate and the actual EGR rate is reduced to zero, and the EGR control valve is controlled based on an EGR control value obtained by adding the EGR correction value to the EGR basic control variable, whereby the actual EGR rate is controlled to the target EGR rate.
According to the first prior art, the high-temperature EGR amount and the low-temperature EGR amount are controlled simply by determining the ratios of the high-temperature EGR amount and the low-temperature EGR amount by searching a map, and then duty-controlling the high-temperature EGR control valve and the low-temperature EGR control valve based on these ratios. Therefore, control accuracy in EGR control is relatively low, which degrades accuracy in control of in-cylinder temperature as well. For these reasons, there is a fear that fuel economy, drivability, and exhaust emission might be all degraded. Further, the conventional EGR control system cannot be applied to an engine, such as a compression ignition combustion engine, which requires highly accurate control of in-cylinder temperature in combustion of a mixture.
According to the second prior art, the internal EGR amount is controlled by changing the intake and exhaust cam phases using the variable valve actuating mechanisms, to thereby change the valve overlap of the intake and exhaust valves. Therefore, in changing the internal EGR amount, it is required to relatively largely change the intake and exhaust cam phases, and it takes time before the intake and exhaust cam phases are actually changed in response to associated control signals after delivery of the signals to the variable valve actuating mechanisms, i.e. the variable valve actuating mechanisms suffer from response delay, so that the control of the internal EGR amount is low in responsiveness, which makes it impossible to promptly control the internal EGR amount to a desired EGR amount.
Further, to control the internal EGR amount, it is necessary to calculate the overlap area between respective non-linear valve lift curves of the intake and exhaust valves, and for accurate calculation of the overlap area, it is required to execute complicated computing operations, such as integral operations, which increases computation load. On the other hand, to lessen the computation load, if the computation of the actual valve overlap area is executed in a simplified manner using e.g. an approximate expression, the computation accuracy is lowered which prevents the internal EGR amount from being accurately controlled.
According to the third prior art, the actual EGR rate is calculated based on an oxygen concentration in the intake pipe detected by the oxygen concentration sensor at the location downstream of the portion of the intake pipe into which the EGR gas is recirculated. In general, however, the oxygen concentration sensor has a characteristic that the resolution of the detected concentration is lowered as the oxygen concentration is higher, and hence the detection accuracy of the sensor is lowered. Therefore, when the EGR rate is low, the actual EGR rate cannot be detected with accuracy, so that the actual EGR rate cannot be accurately controlled to the target EGR rate. Further, it is required to provide the oxygen concentration sensor in the intake pipe only for determining the actual EGR rate, which is disadvantageous in costs.
Further, as is apparent from the above-described method, the EGR control system is effective in performing what is called external EGR in which exhaust gases are recirculated into the intake pipe. However, in performing what is called internal EGR in which part of combustion gases are caused to remain in the combustion chamber without being exhausted therefrom, it is impossible to accurately calculate an internal EGR rate which is a ratio of an amount of combustion gases to a total amount of gases existing in the combustion chamber.